1. Field of the Invention
The present invention relates to techniques for inspecting and qualifying a photo-mask. More specifically, the invention relates to a technique for determining whether or not a photo-mask is acceptable by reconstructing a mask pattern of the photo-mask from a lower resolution image.
2. Related Art
Photolithography is a widely used technology for producing integrated circuits. In this technique, a light source illuminates a photo-mask. The resulting spatially varying light pattern is projected on to a photoresist layer on a semiconductor wafer by an optical system (referred to as an ‘exposure tool’). By developing the 3-dimensional pattern produced in this photoresist layer, a layer in the integrated circuit is created. Furthermore, because there are often multiple layers in a typical integrated circuit, these operations may be repeated using several photo-masks to produce a product wafer.
Unfortunately, as dimensions in integrated circuits steadily become a smaller fraction of the wavelength of the light used to expose images of the photo-mask onto the wafer, the structures in or on the ideal photo-mask (which corresponds to a ‘target mask pattern’) and/or the physical structures in or on the actual photo-mask (which corresponds to a fabricated ‘mask pattern’) bear less and less resemblance to the desired or target pattern at the wafer. These differences between the target mask pattern and the target pattern are used to compensate for the diffraction and proximity effects that occur when light is transmitted through the optics of the exposure tool and is converted into the 3-dimensional pattern in the photoresist.
From a photo-mask or reticle manufacturing standpoint, the increasing dissimilarity between the photo-mask and the corresponding wafer patterns creates a broad new class of problems in photo-mask inspection and qualification. For example, if a defect in a photo-mask is detected, it is often unclear what impact this defect will have on the final pattern in the photoresist. In addition, photo-mask inspection devices often have a different numerical aperture, a different illumination configuration (or ‘source aperture,’ which is also referred to as a ‘source pattern’), and even different light wavelength(s) than those used in the exposure tool. As a consequence, the image measured by an optical photo-mask inspection tool is often neither a perfect replica of the physical photo-mask nor the pattern (i.e., the aerial image) that will be exposed onto the wafer.
One existing approach to these challenges is to use a computer to simulate the resulting wafer pattern based on the optical inspection images of the photo-mask. By comparing simulations of wafer patterns corresponding to the ideal photo-mask (i.e., the target mask pattern) and those associated with an estimate of the actual photo-mask corresponding to the optical inspection images of the photo-mask, the significance of the defect may be determined. However, since the optical inspection images of the photo-mask may not be an accurate representation of the actual photo-mask, errors may be introduced when simulating wafer patterns, and thus, when trying to identify or classify defects. This may further complicate photo-mask inspection and qualification.
Alternatively, a higher-resolution image of the photo-mask than what can typically be obtained using the optical photo-mask inspection tool may be used in the simulations. For example, a spatial variation of a magnitude of the transmittance of the photo-mask may be determined using a scanning electron microscope (SEM). The resulting high-resolution image of the photo-mask (which is sometimes referred to as a ‘critical-dimension scanning-electron-microscope’ or ‘CD-SEM’ image) can provide a more accurate representation of the physical photo-mask than an optical inspection image.
However, measuring a CD-SEM image can be time consuming and complicated. In particular, the electron beam in an SEM often leads to charging of the photo-mask, which can attract dirt or contamination. Furthermore, subsequent cleaning of the photo-mask may produce additional defects in the photo-mask that are not included in the CD-SEM image and, thus, will not be included in the simulations. Therefore, even though a CD-SEM image can provide a more accurate representation of the original physical photo-mask, errors may still be introduced when simulating wafer patterns, and thus, when trying to identify or classify defects.
Hence, what is needed is a photo-mask inspection and qualification technique that overcomes the problems listed above.